A whipsaw or pitsaw was originally a type of saw used in a saw pit, and consisted of a narrow blade held rigid by a frame and called a frame saw or sash saw (see illustrations). This evolved into a 2-3 metre straight, stiff blade without a frame and a handle at each end. It was used close to the felling site to reduce large logs into beams and planks. Sawyers either dug a large pit or constructed a sturdy platform, enabling a two-man crew to saw, one positioned below the log called the pit-man, the other standing on top called the top-man. The saw blade teeth were angled and sharpened as a rip saw so as to only cut on the downward stroke. This arrangement made it easier for the man above to raise the saw, thereby reducing fatigue and backache - the sawyers worked together to raise, lower, and guide the saw. The pitman had to contend with sawdust in his mouth and eyes and the risk of being crushed by a falling log. 
Two men sawing lumber with a pit saw, the log on trestles rather than over a saw pit
A postcard of Chinese sawyers using frame saws around 1900. This method does not need a saw pit.
- The New Shorter Oxford English Dictionary (1993)